Exposure head

ABSTRACT

In the present invention provides, in order to obtain a desired focal depth t in a range α of an acceptable increased amount of beam diameter, an exposure head is designed so that the ratio D/W of an output beam width D to a beam width W at the position where a DMD is placed satisfies the following relational formula. 
     
       
         
           
             
               D 
               W 
             
             ≤ 
             
               
                 
                   
                     α 
                     × 
                     M 
                   
                   
                     2 
                     × 
                     t 
                   
                 
                 - 
                 
                   
                     K 
                     × 
                     λ 
                   
                   a 
                 
               
               θ 
             
           
         
       
     
     In the above formula, parameters are defined as follows.
     λ: the wavelength of laser light   θ: the angle of beam outputted from an illumination light source that is derived by a numerical aperture (NA) of optical fiber according to the following formula
 
θ=sin −1 (NA)
   D: the width of beam outputted from the illumination light source   W: the beam width at the position where the DMD is placed (at the irradiated surface)   a: the size of one pixel on the DMD   K: a coefficient determined by beam characteristics, K=1   M: the magnification of imaging optical system   t: required focal depth   α: acceptable increased amount of beam diameter

This is a divisional of application Ser. No. 10/443,995 filed May 23,2003, now U.S. Pat. No. 6,928,198.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exposure head and in particular to,an exposure head that exposes a photosensitive material with light beamwhich is modulated, according to image data, by a digital micromirrordevice (DMD).

2. Description of the Related Art

Conventionally, a DMD is a mirror device in which a large number ofmicromirrors with angles of their reflection surfaces being variedaccording to control signals are arranged two-dimensionally on asemiconductor substrate such as a silicon or the like. An exposuredevice using this DMD is structured, as shown in FIG. 15, by a lightsource 1 for irradiating laser light, a lens system 2 for collimatingthe laser light irradiated from the light source 1, a DMD 3 disposed ata substantial focus position of the lens system 2 and lens systems 4 and6 for imaging the laser light reflected by the DMD 3 onto a scanningsurface 5. According to such exposure device, each of the micromirrorsfor the DMD 3 is on-off controlled by an unillustrated control devicedepending on a control signal generated according to image data or thelike in order to modulate laser light, whereby image exposure isperformed by the modulated laser light.

The light source 1 is structured as follows. Namely, as shown in FIG.16, a plurality of units each of which includes a single semiconductorlaser 7, a single multi-mode optical fiber 8 and a pair of collimatorlenses 9 for collimating laser light irradiated from the semiconductorlaser 7 to bind on the end surface of the multi-mode optical fiber 8 areplaced and a plurality of the multi-mode optical fibers 8 are bundled,so that a bundled fiber light source is formed.

A laser with around 30 mW of output is usually used for thesemiconductor laser 7. An optical fiber with 50 μm of core diameter, 25μm of clad diameter and 0.2 of NA (numerical aperture) is used for themulti-mode optical fiber 8. Thus, in order to obtain about 1 W ofoutput, 48 (8×6) multi-mode optical fiber units 8 each of which has theabove-described structure must be bundled, and the diameter of luminouspoint is about 1 mm.

According to conventional light sources, however, there arise problemsthat the diameter of luminous point becomes large when a plurality ofoptical fibers are bundled and thus sufficient focal depth cannot beobtained in a case in which exposure heads with high resolution are tobe structured. Especially when only a part of DMD area is used, laserlight must be condensed because the laser light is to be irradiated on anarrow area. As a result, sufficient focal depth cannot be obtained.

There is provided a method in which a focal depth is adjusted byperforming autofocus by moving an imaging lens. Nevertheless, if anautofocus mechanism is provided, demerits such as an increase in costsand deterioration of vibration resistance characteristic may occur.

The invention was developed in order to solve the above-describedproblems and an object of the invention is to provide an exposure headthat is capable of obtaining deep focal depth without providing anautofocus mechanism.

SUMMARY OF THE INVENTION

In order to accomplish the aforementioned object, according to a firstaspect of the present invention, there is provided an exposure headwhich is moved relative to an exposed surface in a direction orthogonalto a predetermined direction, comprising: a laser device which has aplurality of fiber light sources for emitting laser light incident fromoptical-fiber incident ends thereof, from output ends thereof, and inwhich luminous points at the optical-fiber output ends of the pluralityof fiber light sources are arranged; a modulation means which is capableof changing a modulation state of laser light, in accordance with acontrol signal, for each of pixels thereon which have been arranged in atwo dimensional manner; and an optical system for imaging laser lightoutputted from the laser device and modulated at pixel portions of themodulation means onto the exposed surface, wherein parameters defined asfollows satisfy the following formula.

$\frac{D}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{\alpha}}{\theta}$

-   λ: the wavelength of laser light-   θ: the angle of beam outputted from optical fiber that is derived by    a numerical aperture (NA) of optical fiber according to the    following formula    θ=sin⁻¹(NA)-   D: the width of beam outputted from the laser device-   W: the beam width at the position of the modulation means (at the    irradiated surface)-   a: the size of one pixel on the modulation means-   K: a coefficient determined by beam characteristics, K=1-   M: the magnification of imaging optical system-   t: required focal depth-   α: acceptable increased amount of beam diameter

According to the exposure head of this aspect, by designing an exposurehead so that the ratio D/W of the output beam width D to the beam widthW at the position where the DMD is placed satisfies a predeterminedrelational formula in relation to parameters such as the required focaldepth t, the acceptable increased amount a of beam diameter, theoutputted beam angle θ, the imaging magnification M of imaging opticalsystem, the wavelength λ of laser light, the characteristic coefficientK and the size a of one pixel on the modulation means (a spatial lightmodulation element), the exposure head which has the desired focal deptht in the range of the acceptable increased amount α of beam diameter canbe realized. Namely, deep focal depth can be obtained without providingan autofocus mechanism.

According to a second aspect of the invention, a DMD (digitalmicromirror device) can be used as the modulation means in the exposurehead.

According to a third aspect of the present invention, in order toaccomplish high intensity laser device for the exposure head, an opticalfiber in which a core diameter thereof is uniform and a clad diameterthereof at the output end is smaller than that at the incident end ispreferably used as the fiber light source.

According to a fourth aspect of the invention, a fiber light source (amultiplexing laser light source) that multiplexes a plurality of laserlights to make the resultant multiplexed light incident onto each of theoptical fibers is preferably used.

Because of using the multiplexing laser light source, fiber lightsources structuring the laser device may have high outputs, and suchhigh output can be obtained by less number of fibers. Consequently,higher intensity and reduction in costs can be accomplished.

According to a fifth aspect of the invention, the exposure head ispreferably structured so that a value represented by the followingformula in the predetermined direction is substantially equal to a valuerepresented by the following formula in a direction orthogonal to thepredetermined direction.

$\frac{\theta \times D}{W} + \frac{K \times \lambda}{a}$

In a case in which the modulation means is a DMD, for example, the aboveformula represents the divergence angle for light reflected by the DMD.If the value in the longer side direction of the DMD is substantiallyequal to the value in the shorter side direction thereof, a focal depthin the longer side direction of the DMD may be substantially equal to afocal depth in the shorter side direction thereof. As a result, exposurecan be performed with high precision.

According to a sixth aspect of the invention, the exposure head of theinvention may comprise a laser device in which a plurality of luminouspoints are arranged with predetermined intervals therebetween. Namely,the exposure head which is moved relative to an exposed surface in adirection orthogonal to a predetermined direction, comprises a laserdevice in which a plurality of luminous points are arranged in apredetermined direction with predetermined intervals therebetween; amodulation means which is capable of changing a modulation state oflaser light, in accordance with a control signal, for each of pixelsthereon which have been arranged in a two dimensional manner; and anoptical system for imaging laser light outputted from the laser deviceand modulated at pixel portions of the modulation means onto the exposedsurface, wherein parameters defined as follows satisfy the followingformula.

$\frac{D_{A}}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{a}}{\theta_{A}}$

-   λ: the wavelength of laser light-   θ_(A): the angle of beam outputted from luminous points-   D_(A): the total width of beams outputted from all luminous points-   W: the beam width at the position of the modulation means (at the    irradiated surface)-   a: the size of one pixel on the modulation means-   K: a coefficient determined by beam characteristics, K=1-   M: the magnification of imaging optical system-   t: required focal depth-   α: acceptable increased amount of beam diameter

It should be noted that a DMD (digital micromirror device can be used asthe modulation means in the aforementioned exposure head).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the schematic structure of an exposurehead relating to a first embodiment.

FIG. 2 is a partial enlarged view illustrating the structure of adigital micromirror device (DMD).

FIGS. 3A and 3B are explanatory views for explaining the operations ofthe DMD.

FIG. 4 is a perspective view illustrating the structure of anillumination light source.

FIG. 5A is a view illustrating the example of the area of the DMD used.

FIG. 5B is a view illustrating another example of the area of the DMDused.

FIG. 6A is a side view in the case in which an appropriate area of DMDis used.

FIG. 6B is a cross-sectional view in a sub-scanning direction along anoptical axis shown in FIG. 6A.

FIG. 7 is an explanatory view for explaining parameters used in theinvention.

FIG. 8A is a plan view illustrating the arrangement of luminous pointsat the output end of the illumination light source used in the firstembodiment, and the output beam width thereof.

FIG. 8B is a plan view illustrating the arrangement of luminous pointsat the output end of the illumination light source relating to acomparative example, and the output beam width thereof.

FIG. 9 is a plan view illustrating the arrangement of luminous points atthe output end of the illumination light source used in a secondembodiment, and the output beam width thereof.

FIG. 10 is a plan view illustrating the arrangement of luminous pointsat the output end of the illumination light source used in a thirdembodiment, and the output beam width thereof.

FIG. 11 is a plan view illustrating the structure of a multiplexinglaser light source.

FIG. 12 is a plan view illustrating the structure of a laser module.

FIG. 13 is a side view of the structure of the laser module shown inFIG. 12.

FIG. 14 is a partial side view of the structure of the laser moduleshown in FIG. 12.

FIG. 15 is a cross-sectional view along an optical axis, illustratingthe structure of an exposure head that uses the DMD as a spatial lightmodulation element.

FIG. 16 is a cross-sectional view along an optical axis, illustratingthe structure of a conventional fiber light source.

FIGS. 17A and 17B are explanatory views for explaining parameters in thecase of using a multi-cavity laser as the illumination light source.

FIG. 18 is an explanatory view for explaining parameters in the case ofusing a multi-cavity laser array as the illumination light source.

FIG. 19 is a side view showing a light source in which broad stripelaser is used.

FIG. 20 is a plan view of the light source of FIG. 19.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of an exposure head of the present invention will bedescribed hereinafter in detail with reference to the drawings.

FIRST EMBODIMENT

[Structure of Exposure Head]

An exposure head relating to the present embodiment includes, as shownin FIG. 1, a digital micromirror device (DMD) 50 serving as a modulationmeans (a spatial light modulation element) for modulating, on apixel-by-pixel basis, light beam incident thereon according to imagedata. The DMD 50 is connected to an unillustrated controller thatincludes a data processing section and a mirror drive control section.In the data processing section of the controller, a control signal fordrive-controlling each of micromirrors within an area of the DMD 50 tobe controlled, of the exposure head, is generated on a basis of inputtedimage data. The area to be controlled will be described later. In themirror drive control section, on a basis of the control signal generatedin the image data processing section, the angle of the reflectionsurface of each of the micromirrors in the DMD 50 is controlled at theexposure head. The control of the angle of the reflection surface willbe described later.

Successively disposed at the light-incident side of the DMD 50 are anillumination light source 66 with a laser-outputting portion in whichtwo rows of optical-fiber output end portions (luminous points) arearranged along a direction corresponding to a longer side of the DMD 50,a lens system 67 for correcting the laser light outputted from theillumination light source 66 to condense on the DMD, and a mirror 69 forreflecting the laser light which has transmitted through the lens system67 toward the DMD 50.

The lens system 67 is formed of a lens system for making laser lightoutputted from the illumination light source 66 parallel light, a lenssystem for correcting the parallel laser light so as to have uniformlight amount distribution and a condenser lens system for condensing thelaser light with the light amount distribution thereof having beencorrected onto the DMD. Lens systems 54 and 58 for imaging the laserlight reflected by the DMD 50 onto a scanning surface (exposed surface)56 are disposed at the light-reflecting side of the DMD 50. The lenssystems 54 and 58 are disposed so that the DMD 50 is conjugated with theexposed surface 56.

The DMD 50 is, as shown in FIG. 2, a mirror device in which micromirrors62 are placed on an SRAM cell (memory cell) 60 by being supported bypoles and a large number (e.g., 600×800) of micromirrors constitutingpixels are arranged in a lattice manner. When a digital signal iswritten into the SRAM cell 60 of the DMD 50, each of the micromirrors 62supported by the poles is diagonally inclined with respect to the sideof substrate on which the DMD 50 is placed within the range of ±βdegrees (e.g., ±10 degrees). On-off control for each of the micromirrors62 is performed by the unillustrated controller connected to the DMD 50.

FIG. 3A shows the state that the micromirror 62 is in an “on” state(which state will be referred to as “an on state”, hereinafter) andinclined by ±β degrees. FIG. 3B shows the state that the micromirror 62is in an “off” state (which state will be referred to as “an off state”,hereinafter) and inclined by −β degrees. Thus, by controllinginclination of the micromirrors 62 on pixels of the DMD 50 as shown inFIG. 2, according to an image signal, light incident on the DMD 50 isreflected in directions that the micromirrors 62 are inclined.

As shown in FIG. 4, the illumination light source 66 has a plurality of(six in this example) laser modules 64. One end of a multi-mode opticalfiber 30 is connected to each of the laser modules 64. The other end ofthe multi-mode optical fiber 30 is drawn from a package for the lasermodule 64. In this way, a laser-outputting portion 68 in which sixluminous points are arranged so that three points are arranged in alonger side direction of the DMD 50 and two points are arranged in ashorter side direction thereof is structured. When the output of each ofthe luminous points for the illumination light source 66 is 180 mW, theoutput of the laser-outputting portion 68 in which six luminous pointsare arranged is about 1 W (=180 mW×6).

[Operation of Exposure Head]

Next, the operation of the above-described exposure head will bedescribed.

When laser light is irradiated from the illumination light source 66 tothe DMD 50, each of the micromirrors of the DMD 50 is on-off controlledby the unillustrated controller. Laser light reflected when themicromirrors of the DMD 50 are in an on state is imaged onto the exposedsurface 56 of a photosensitive material by the lens systems 54 and 58.Laser light outputted from the illumination light source 66 is turned onor off on a pixel-by-pixel basis, so that the photosensitive material isexposed with light on a pixel unit (exposure area 168) basis which unithaving the substantially same number of pixels as pixels used in the DMD50. The photosensitive material is moved by unillustrated movement meansat a fixed speed, whereby the photosensitive material is sub-scanned bythe exposure head in the direction opposite to the direction in whichthe photosensitive material is moved and. As a result, an exposed area170 is formed in a band shape.

600 micromirror rows are arranged in the DMD 50 in a sub-scanningdirection. In each row, 800 micromirrors are arranged in a main scanningdirection. Control may be performed so that only a part of themicromirror rows (e.g., only 800×50 rows) is driven by the controller.As shown in FIG. 5A, the micromirror row placed at the central portionof the DMD 50 may be used. Alternatively, the micromirror row placed atthe end portion of the DMD 50 may be used as shown in FIG. 5B. Further,if a part of the micromirrors is damaged, micromirrors rows withoutdamages may be used. Namely, micromirror rows used may be appropriatelychanged depending on conditions.

There is a limit in data processing speed for the DMD 50. A modulationspeed per line is determined in proportion to the number of pixels used.Thus, the modulation speed per line is increased by using only a part ofmicromirror rows. In the case of an exposure method that an exposurehead is successively moved relative to an exposure surface, all ofpixels in a sub-scanning direction need not to be used.

For example, if 300 micromirror rows of 600 micromirror rows are used,modulation per line may be performed twice faster as compared to thecase of using all 600 micromirror rows. If 200 micromirror rows of 600micromirror rows are used, the modulation per line may be performedthree times faster as compared to the case of using all 600 micromirrorrows. Namely, exposure can be completed for 17 seconds upon an area with500 mm of sub-scanning direction width. Further, if 100 micromirror rowsof 600 micromirror rows are used, the modulation per line may beperformed six times faster as compared to the case of using all 600micromirror rows.

Namely, exposure can be completed for nine seconds upon the area with500 mm of sub-scanning direction width.

The number of micromirror rows used, i.e., the number of micromirrorsarranged in a sub-scanning direction is preferably in the range of 10 to200 and more preferably in the range of 10 to 100. The area of amicromirror corresponding to a pixel is 15 μm×15 μm. When convertinginto the area used of the DMD 50, areas of 12 mm×150 μm to 12 mm×3 mmare preferable and areas of 12 mm×150 μm to 12 mm×1.5 mm are morepreferable.

When the number of micromirror rows used is in the aforementionedranges, as shown in FIGS. 6A and 6B, laser light outputted from theillumination light source 66 may be made into substantial parallel lightby the lens system 67 and then the parallel light may be irradiated ontothe DMD 50. The area on which the DMD 50 irradiates laser lightpreferably coincides the area used in the DMD 50. If the irradiationarea is larger than the area used, the efficiency of utilizing laserlight is decreased. The sub-scanning direction diameter of light beamcondensed onto the DMD 50 must be decreased by the lens system 67according to the number of micromirrors arranged in a sub-scanningdirection. If the number of micromirror rows used is smaller than 10, itis not preferable because the angle of luminous flux incident on the DMD50 becomes large and a focal depth of light beam on the scanning surface56 is decreased. 200 or smaller of micromirror rows are preferably usedin view of modulation speed. The DMD is a reflection type spatialmodulation element (modulation means). FIGS. 6A and 6B, however, showexploded views in order to explain optical relationships.

[Derivation of Relational Formulae]

According to the exposure head relating to the present embodiment, asshown in FIG. 7, parameters θ, D, φ, ψ, W, Δy and Δz are defined.Namely, θ indicates the angle of beam outputted from each of opticalfibers arranged in the beam-outputting portion of the illumination lightsource 66. D indicates the width of beam outputted from the illuminationlight source 66. φ indicates the angle formed by light outputted fromthe central luminous point among luminous points arranged in theillumination light source 66 along a predetermined direction and lightoutputted from the luminous point placed at the end of the illuminationlight source 66. ψ indicates a divergence angle formed by diffraction oflight reflected by the DMD 50. W indicates a beam width at the positionwhere the DMD 50 is placed (at the irradiated surface). Δz indicates anerror in a direction of focal depth and Δy indicates an increased amountof beam diameter (one side) when Δz indicates the focal depth directionerror.

According to the present embodiment, in order to obtain a desired focaldepth t in the range of the acceptable increased amount α of beamdiameter, the exposure head is designed so that the ratio D/W of theoutputting beam width D to the beam width W at the position where theDMD is placed satisfies the following relational formula (A).

$\begin{matrix}{\frac{D}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{a}}{\theta}} & (A)\end{matrix}$

In the above formula, parameters are defined as follows.

-   λ: the wavelength of laser light-   θ: the angle of beam outputted from the illumination light source    that is derived from numerical aperture (NA) of optical fiber    according to the following formula    θ=sin⁻¹(NA)-   D: the width of beam outputted from the illumination light source-   W: the beam width at the position DMD is placed (at the irradiated    surface)-   a: the size of one pixel on DMD-   K: a coefficient determined by beam characteristics, K=1-   M: the magnification of imaging optical system

A method for deriving the aforementioned relational formula will bedescribed with reference to FIG. 7. The lens system 67 makes light fromthe illumination light source 66 substantial parallel light and isdisposed so that luminous points at the illumination light source 66substantially evenly illuminate the entire area used in the DMD 50.Luminous fluxes outputted from the luminous points illuminate the entirearea used in the DMD 50. Thus, even if a part of laser modulesstructuring the illumination light source 66 is broken, a light amountdistribution of laser light irradiated onto the DMD 50 cannot be uneven.Under such conditions, the angle φ (rad) formed by light outputted fromthe central luminous point among a plurality of luminous points arrangedin a predetermined direction and light outputted from the luminous pointplaced at the end portion of the light source 66 is represented by thefollowing formula by using the angle θ of outputted beam (rad), theoutput beam width D (mm) and the beam width W (mm) at the position wherethe DMD is placed.

$\phi = \frac{\theta \times D}{W}$

Beam to be irradiated onto the DMD 50 is substantially parallel beam buthas the angle of ±φ (rad). Light reflected by the DMD 50 is diverged bya divergence angle ψ (rad) because of diffraction effect caused byinfluence of the size of pixels in the DMD 50. Accordingly, thedivergence angle of the light reflected by the DMD 50 is given by thefollowing formula.φ+ψ

A diffraction divergence angle ψ (rad) when a pitch of diffractiongrating (size of a pixel in the DMD 50) is a (μm) and the wavelength ofincident laser light is λ (μm) is given by the following formula. K is acoefficient determined by beam characteristics and is usually 1.

$\psi = \frac{K \times \lambda}{a}$

When the imaging magnification for the imaging optical systems 54 and 58is indicated by M, the divergence angle for beam illuminating onto theexposed surface 56 is given by the following formula.

$\frac{\left( {\phi + \psi} \right)}{M}$

When the focal depth direction error is indicated by Δz, therelationship between the increased amount Δy of beam diameter (one side)and the error Δz is given by the following formula by using thedivergence angle for beam illuminating onto the exposed surface 56.

${\Delta\; y} = \frac{\left( {\phi + \psi} \right) \times \Delta\; z}{M}$$\frac{\Delta\; y}{\Delta\; z} = \frac{\frac{\theta \times D}{W} + \frac{K \times \lambda}{a}}{M}$

Note that it is assumed that the required focal depth is indicated by t(μm) and the acceptable increased amount of beam diameter is indicatedby α (μm). When the following formula is satisfied (i.e., when t≦Δz),the required focal depth t can be obtained in the range of theacceptable increased amount a of beam diameter. Here, α=2×Δy.

$\frac{a}{t} \geq {2 \times \frac{\frac{\theta \times D}{W} + \frac{K \times \lambda}{a}}{M}}$

By transforming the above formula, the relational formula (A) can beobtained.

Namely, by designing the exposure head so that the ratio D/W of theoutput beam width D to the beam width W at the position where the DMD isplaced satisfies the above relational formula (A) in relation to theparameters such as the required focal depth t, the acceptable increasedamount a of beam diameter, the angle θ of outputted beam, the imagingmagnification M of imaging optical systems, the wavelength λ of laserlight, the characteristic coefficient K and the size “a” of a pixel inthe DMD, the exposure head which has a desired focal depth t in therange of the acceptable increased amount α of beam diameter can berealized.

In the above-described exposure head, it is assumed that the wavelengthλ of laser light is 0.4 μm, the size a of one pixel in the DMD 50 is 20μm, the magnification M for imaging optical systems is 1, thecharacteristic coefficient K is 1 and the acceptable increased amount αof beam diameter is 2 μm. The drive area in the DMD 50 is 16 mm×1 mm(800 pixels×50 pixels) . Micromirrors corresponding to 800 pixels areused in a longer side direction of the DMD. Micromirrors correspondingto 50 pixels are used in a shorter side direction of the DMD.Correspondingly, the beam width W in the longer side direction of theDMD at the position where the DMD is placed is 17.6 mm and the beamwidth W in the shorter side direction of the DMD is 1.1 mm.

Under such conditions, in order to satisfy the above relational formula,the clad diameter of the multi-mode optical fiber 30 is to be 60 μm, thecore diameter is to be 25 μm and NA is 0.2. Further, the output beamwidth D in the longer side direction of the DMD is to be 0.145 mm, theoutput beam width D in the shorter side direction of the DMD is to be0.085 mm and the angle θ of outputted beam is to be 0.2 rad, with thebeam being outputted from the illumination light source 66 in whichthree luminous points are arranged in the longer side direction of theDMD and two luminous points are arranged in the shorter side directionof the DMD, as shown in FIG. 8A. The output beam width D in the shorterside direction of the DMD is 0.085 mm and the angle θ of outputted beamis 0.2 rad. Thus, as shown in Table 1, long focal depths such as 47 μmin the longer side direction of the DMD and 30 μm in the shorter sidedirection of the DMD can be realized.

TABLE 1 longer side direction of DMD shorter side direction of DMD λ 0.4μm 0.4 μm θ 0.2 rad 0.2 rad D 0.145 mm 0.085 mm W 17.6 mm 1.1 mm WD 16mm (800 pixels) 1 mm (50 pixels) a 20 μm 20 μm M 1 1 α 2 μm 2 μmObtained 47 μm 30 μm focal depth

On the other hand, in a case of using a fiber light source structured bybinding light outputted from a semiconductor laser with around 30 mW ofoutput into a multi-mode optical fiber which has 50 μm of core diameter,125 μm of clad diameter and 0.2 of NA, in order to obtain about 1 W ofoutput, 8×6, i.e., 48 multi-mode optical fibers must be bundled as shownin FIG. 8B. In this case, the output beam width D in the longer sidedirection of the DMD is 0.925 mm and the output beam width D in theshorter side direction of the DMD is 0.675 mm. In order to obtain 17.6mm of the beam width W in the longer side direction of the DMD at theposition where the DMD 50 is placed and 1.1 mm of the beam width W inthe shorter side direction of the DMD, as shown in the Table 2, thefocal depth in the longer side direction of the DMD is 32 μm. However,the above relational formula is not satisfied in the case of the focaldepth in the shorter side direction of the DMD, and the focal depth is 7μm. Thus, an autofocus mechanism is required.

TABLE 2 longer side direction of DMD shorter side direction of DMD λ 0.4μm 0.4 μm θ 0.2 rad 0.2 rad D 0.952 mm 0.675 mm W 17.6 mm 1.1 mm WD 16mm (800 pixels) 1 mm (50 pixels) a 20 μm 20 μm M 1 1 α 2 μm 2 μmObtained 32 μm 7 μm focal depth

As described above, according to the present embodiment, by designingthe exposure head so that the ratio D/W of the output beam width D tothe beam width W at the position where the DMD is placed satisfies thepredetermined relational formula in relation to parameters such as therequired focal depth t, the acceptable increased amount α of beamdiameter, the angle θ of outputted beam, the imaging magnification M forthe imaging optical systems, the wavelength λ of laser light, thecharacteristic coefficient K and the size “a” of a pixel in the DMD, theexposure head which has the desired focal depth t in the range of theacceptable increased amount α of beam diameter can be realized. Namely,deep focal depth can be obtained without providing an autofocusmechanism.

According to the present embodiment, a high intensity illumination lightsource in which luminous points at optical-fiber output end portions ofmultiplexing laser light sources are arranged in a bundled manner isused for a light source for illuminating the DMD. Thus, an exposure headwith high output and deep focal depth can be realized. Further, sincethe output of each of luminous points becomes large, the number offibers needed in order to obtain a desired output is decreased,resulting in a decrease in costs for the exposure head.

SECOND EMBODIMENT

According to an exposure head relating to a second embodiment, thearrangement of luminous points in an illumination light source ischanged so that a divergence angle (i.e., φ+ψ) for light reflected by aDMD that is represented by the following formula in a longer sidedirection of the DMD is equal to the divergence angle in the shorterside direction of the DMD. Because the second embodiment has the samestructures as those in the first embodiment except for this point,descriptions thereof will be omitted.

$\frac{\theta \times D}{W} + \frac{K \times \lambda}{a}$

The drive area for the DMD 50 is 16 mm×1 mm (800 pixels×50 pixels).Similarly to the first embodiment, it is assumed that the beam width Win the longer side direction of the DMD at the position where the DMD 50is placed is 17.6 mm and the beam width W in the shorter side directionof the DMD is 1.1 mm, in accordance with a ratio 16:1 of the longer sidevs. the shorter side of the drive area for the DMD 50. In this case,when the ratio of the width D of beam outputted from the illuminationlight source 66 in the longer side direction of the DMD to the beamwidth in the shorter side direction of the DMD is 16:1, a condensingratio by the lens system 67 in the longer side direction of the DMD issubstantially equal to a condensing ratio by the lens system 67 in theshorter side direction of the DMD. Thus, the divergence angle for lightreflected by the DMD in the longer side direction of the DMD issubstantially equal to the divergence angle for light reflected by theDMD in the shorter side direction.

For example, when the illumination light source 66 is formed of lowintensity fiber light sources each of which has 30 mW of output at itsluminous point, 48 multi-mode optical fibers 30 must be bundled in orderto obtain about 1 W of output. Assuming that the number of opticalfibers to be bundled is 48, 48 luminous points at the laser-outputtingportion of the illumination light source 66 may be arranged so that 48luminous points are arranged in the longer side direction of the DMD andone luminous point is arranged in the shorter side direction of the DMD.Alternatively, 48 luminous points may be arranged so that 24 luminouspoints are arranged in the longer side direction of the DMD and twoluminous points are arranged in the shorter side direction of the DMD.As a result, the ratio of the output beam width D in the longer sidedirection of the DMD to the output beam width D in the shorter sidedirection may made close to 16:1.

It is assumed that the clad diameter of the multi-mode optical fiber 30is 125 μm, the core diameter thereof is 50 μm and NA is 0.2. Then, asshown in FIG. 9, the width D of beam outputted in the longer sidedirection of the DMD from the illumination light source 66, in which 48luminous points are arranged as two rows in the longer side direction ofthe DMD, is 2.9 mm and the output beam width D in the shorter sidedirection of the DMD is 0.175 mm. If the angle θ for outputted beam is0.2 rad, as shown in the following Table 3, long focal depths such as 20μm in the longer side direction of the DMD and 20 μm in the shorter sidedirection thereof can be realized.

TABLE 3 Case of using low intensity fiber light source Longer sidedirection of DMD Shorter side direction of DMD λ 0.4 μm 0.4 μm θ 0.2 rad0.2 rad D 2.9 mm 0.175 mm W 17.6 mm 1.1 mm WD 16 mm (800 pixels) 1 mm(50 pixels) a 20 μm 20 μm M 1 1 α 2 μm 2 μm Obtained 20 μm 20 μm focaldepth

As described above, according to the second embodiment, as in the firstembodiment, deep focal depth can be obtained without providing anautofocus mechanism. Further, a focal depth obtained in the longer sidedirection of the DMD is substantially equal to a focal depth obtained inthe shorter side direction and thus exposure can be performed with highprecision.

THIRD EMBODIMENT

An exposure head relating to a third embodiment has the same structuresas those of the first embodiment except that, as in the secondembodiment, the arrangement of luminous points in an illumination lightsource is changed so that the divergence angle for light reflected bythe DMD in the longer side direction of the DMD is equal to thedivergence angle for light reflected by the DMD in the shorter sidedirection thereof and the illumination light source is formed of highintensity fiber light sources. Thus, detailed descriptions thereof willbe omitted.

It is assumed that, for example, that the illumination light source 66is formed of high intensity fiber light sources each of which has 180 mWof output at its luminous point. In order to obtain about 1 W of output,six multi-mode optical fibers 30 must be arranged. When the number ofoptical fibers to be arranged is six, as shown in FIG. 10, six luminouspoints at the laser-outputting portion of the illumination light source66 may be arranged so that six luminous points are arranged in thelonger side direction of the DMD and one luminous point is arranged inthe shorter side direction thereof. As a result, the ratio of the outputbeam width D in the longer side direction of the DMD to the output beamwidth D in the shorter side direction of the DMD may be made close to16:1.

When the clad diameter of the multi-mode optical fiber 30 is 60 μm, thecore diameter thereof is 25 μm and NA is 0.2, the width D of beamoutputted in the longer side direction of the DMD from the illuminationlight source 66, in which six luminous points are arranged in the longerside direction of the DMD and one luminous point is arranged in theshorter side direction thereof is 0.325 mm, and the width D of beamoutputted in the shorter side direction of the DMD from the sameillumination light source 66 is 0.025 mm. If the angle θ for outputtedbeam is 0.2 rad, as shown in the following Table 4, long focal depthssuch as 44 μm in the longer side direction of the DMD and 41 μm in theshorter side direction thereof can be realized.

TABLE 4 Case of using high intensity fiber light source longer sidedirection of DMD shorter side direction of DMD λ 0.4 μm 0.4 μm θ 0.2 rad0.2 rad D 0.325 mm 0.025 mm W 17.6 mm 1.1 mm WD 16 mm (800 pixels) 1 mm(50 pixels) a 20 μm 20 μm M 1 1 α 2 μm 2 μm Obtained 44 μm 41 μm focaldepth

As described above, according to the third embodiment, as in the firstembodiment, deep focal depth can be obtained without providing anautofocus mechanism. Especially according to the present embodiment, ahigh intensity illumination light source in which luminous points at theoptical-fiber output end portions of multiplexing laser light sourcesare arranged in a bundled manner is used for a light source forilluminating onto the DMD. As a result, an exposure head with highoutput and deep focal depth can be realized.

Further, a focal depth in the longer side direction of the DMD issubstantially equal to a focal depth in the shorter side directionthereof, and thus exposure can be performed with high precision.

[Structure of Laser Module]

The laser module 64 formed of a high intensity fiber light source isstructured by, for example, a multiplexing laser light source as shownin FIG. 11. The multiplexing laser light source is formed of a pluralityof (e.g., seven) lateral multi-mode or single-mode GaN-basedsemiconductor laser chips LD1, LD2, LD3, LD4, LD5, LD6 and LD7 that arearranged on a heat block 10 and fixed thereto, collimator lenses 11, 12,13, 14, 15, 16 and 17 provided so as to respectively correspond to theGaN-based semiconductor lasers LD1 to LD7, a condenser lens 20 and amulti-mode optical fiber 30. The number of the semiconductor lasers isnot limited to seven.

The GaN-based semiconductor lasers LD1 to LD7 have a common oscillationwavelength (e.g., 405 nm) and a common maximum output (e.g., 100 mW inthe case of multi-mode laser and 30 mW in the case of single-modelaser). Lasers with 350 nm to 450 nm of wavelength range and oscillationwavelength other than 405 nm may be used as the GaN-based semiconductorlasers LD1 to LD7.

The multiplexing laser light source with the above-described structureis accommodated together with other optical elements within a box-shapedpackage 40 with the top surface thereof being opened as shown in FIGS.12 and 13. The package 40 has a package cover 41 formed so as to closethe open surface. A sealing gas is introduced subsequent to a degassingtreatment and the opening of the package 40 is closed by the packagecover 41. As a result, the multiplexing laser light source ishermetically sealed within the closed space (sealed space) formed by thepackage 40 and the package cover 41.

A base plate 42 is fixed to the bottom surface of the package 40. Theheat block 10, a condenser lens holder 45 for holding the condenser lens20 and a fiber holder 46 for holding the incident end portion of themulti-mode optical fiber 30 are mounted on the top surface of the baseplate 42. The output end portion of the multi-mode optical fiber 30 isdrawn outside the package from an opening formed at the wall surface ofthe package 40.

A collimator lens holder 44 is mounted to the side surface of the heatblock 10 and holds the collimator lenses 11 to 17. Openings are formedat the lateral wall surface of the package 40 and wirings 47 forsupplying drive current to the GaN-based semiconductor lasers LD1 to LD7are drawn outside the package through the openings.

Referring to FIG. 13, in order to avoid complication, the referencenumeral is attached only to the GaN-based semiconductor laser LD7 amongthe plurality of GaN-based semiconductor lasers and to the collimatorlens 17 among the plurality of collimator lenses.

FIG. 14 shows a front view of the portion the collimator lenses 11 to 17are mounted. Each of the collimator lenses 11 to 17 is formed in anelongated configuration which would be obtained by cutting, by planesparallel to each other, an area of circular lens having an asphericsurface and including an optical axis. The elongated collimator lens maybe formed by molding, for example, a resin or an optical glass. Thecollimator lenses 11 to 17 are disposed, close to each other, in adirection that luminous points are arranged, so that the longitudinaldirections thereof are orthogonal to a direction in which luminouspoints of the GaN-based semiconductor lasers LD1 to LD7 are arranged(i.e., right and left direction on the page surface of FIG. 14).

Lasers that have active layers with 2 μm of emission width and emitlaser beams B1 to B7 for example with 10° of divergence angle in adirection parallel to the active layers and 30° of divergence angle in adirection orthogonal to the active layers are used as the GaN-basedsemiconductor lasers LD1 to LD7. Such GaN-based semiconductor lasers LD1to LD7 are disposed so that their luminous points are arranged in a rowin a direction parallel to the active layers.

Accordingly, the laser beams B1 to B7 respectively emitted from theluminous points are incident on the elongated collimator lenses 11 to 17so that a direction in which the divergence angle is relatively largecoincides with the longitudinal direction of the collimator lenses and adirection in which the divergence angle is relatively small coincideswith the widthwise direction thereof (i.e., “the widthwise direction”represents a direction orthogonal to the longitudinal direction). Eachof the collimator lenses 11 to 17 has a width of 1.1 mm and a length of4.6 mm. The horizontal diameter of laser beams B1 to B7 incident on suchcollimator lenses 11 to 17 is 0.9 mm and the vertical diameter thereofis 2.6 mm. Each of the collimator lenses 11 to 17 has a focal distancef1=3 mm, NA=0.6 and a lens arrangement pitch=1.25 mm.

The condenser lens 20 is obtained in an elongated configuration bycutting, by planes parallel to each other, an area of circular lenshaving an aspheric surface and including an optical axis. The condenserlens 20 is formed so as to be long in the direction in which thecollimator lenses 11 to 17 are arranged, i.e., in a horizontal directionand short in a direction orthogonal to the horizontal direction. Thecondenser lens 20 has a focal distance f2=23 mm and NA=0.2. Also, thecondenser lens 20 may be formed by molding, for example, a resin or anoptical glass.

According to such laser module, laser beams B1, B2, B3, B4, B5, B6 andB7 emitted as divergent light from the GaN-based semiconductor lasersLD1 to LD7 constituting multiplexing laser light sources for the fiberarray light source 66 are made into parallel lights by the correspondingcollimator lenses 11 to 17. The parallel laser beams B1 to B7 arecondensed by the condenser lens 20 and converged onto the incident endsurface of a core 30 a of the multi-mode optical fiber 30.

According to the present embodiment, a condensing optical system isformed by the collimator lenses 11 to 17 and the condenser lens 20, anda multiplexing optical system is formed by the condensing optical systemand the multi-mode optical fiber 30. The laser beams B1 to B7 condensedas described above by the condenser lens 20 are incident on the core 30a of the multi-mode optical fiber 30, proceed within the optical fiberand are multiplexed into a laser beam B. Then, the laser beam B outputsfrom the optical fiber 31 coupled to the light-outputting end portion ofthe multi-mode optical fiber 30 outputs.

When the efficiency of coupling the laser beams B1 to B7 to themulti-mode optical fiber 30 is 0.85 and the GaN-based semiconductorlasers LD1 to LD7 have 30 mW of outputs in each of the laser modules, amultiplexing laser beam B with 180 mW (=30 mW×0.85×7) can be obtainedfor each of optical fibers 31 arranged in an array.

Examples that laser light emitted from a plurality of semiconductorchips is incident on an optical fiber have been described above.However, alternatively, laser light emitted from a single broad stripelaser device with stripe-shaped emission area may be incident on anoptical fiber.

A modified example of using a broad stripe laser will be describedhereinafter.

As shown in FIGS. 19 and 20, in accordance with the present embodiment,a broad stripe laser 180 is used as an illumination light source. FIG.19 is a side view of a device seen from a side. FIG. 20 is a plan viewof the device seen from above.

A first cylindrical lens 182, a second cylindrical lens 184, a thirdcylindrical lens 186 and a fourth cylindrical lens 188 are disposed inthis order from the side of the broad stripe laser 180, between thebroad stripe laser 180 and one end of a multi-mode optical fiber 30.

As shown in FIGS. 19 and 20, the broad stripe laser 180 has an emissionarea (not shown) which is long in a horizontal direction.

In accordance with the broad stripe laser 180 of the present embodiment,a width (in the vertical direction) of the emission layer thereof is 0.5μm, a length (in the horizontal direction) of the emission layer is 30μm and a wavelength for light beam is 400 to 420 nm.

It should be noted that NA of the beam in the vertical direction isconverted so as to be less than NA of the multi-mode optical fiber 30 bythe first cylindrical lens 182 and the third cylindrical lens 186.

Further, light beam is imaged onto one end of the multi-mode opticalfiber 30 at a magnification of one (i.e., ×1) in the horizontaldirection by the second cylindrical lens 184 and the fourth cylindricallens 188. (Namely, the horizontal-direction length of the emissionlayer, which is 30 μm, is imaged onto the fiber core portion.) Inaccordance with the present embodiment, beams from the broad stripelaser 180 with an output of the emission point thereof being 200 mW areconverged so as to be incident upon one multi-mode optical fiber 30.

If the “light-yield” efficiency is 90%, 180 mW of the light source canbe obtained.

Although the example of coupling a fiber having a clad diameter of 60 μmhas been described above, by making beams from the broad stripe laser180 be directly incident on the multi-mode optical fiber 30 having aclad diameter of 60 μm and a core diameter of 50 μm, the beams havingthe original diameter may be used, as it is, as an output end.

It is preferable that a width of the broad stripe of the broad stripelaser 180 is 5 to 20 μm, NA of the multi-mode optical fiber 30 is 0.15to 0.3 and the diameter of the fiber core is 10 to 80 μm becausecoupling efficiency of the fiber is then improved.

Although the beam shaping optical system is structured by using thecylindrical lenses, the beam shaping optical system may be structured byusing optical components other than the cylindrical lenses.

[Light Source with Plurality of Luminous Points]

There has been described an example in which a fiber bundle lightsource, in which optical fibers from a plurality of fiber light sourcesare bundled, are used as an illumination light source. However, a laserarray in which a plurality of semiconductor laser chips are arranged ona heat block in a predetermined direction with predetermined intervalstherebetween or a multi-cavity laser chip in which a plurality ofluminous points are arranged in a predetermined direction atpredetermined intervals therebetween may be used as the illuminationlight source. In the case of the multi-cavity laser, luminous points canbe arranged with excellent positional precision as compared to the caseof arranging semiconductor laser chips.

An example for using the multi-cavity laser as the illumination lightsource will be described with reference to FIGS. 17A and 17B. As shownin FIG. 17A, in the multi-cavity laser 110, a large number of luminouspoints 110 a are arranged along a predetermined direction withpredetermined intervals therebetween. A total width of beams outputtedfrom all luminous points for the illumination light source under suchstate is indicated by D_(A) (mm). As shown in FIG. 17B, an angle of beamoutputted from a luminous point is indicated by θ_(A) (rad). In theabove relational formula (A), when θ_(A) is inputted instead of theangle θ of beam outputted from optical fiber and the total output beamwidth D_(A) is inputted instead of the width D of beam outputted fromlaser device, the following relational formula (B) is derived.

$\frac{D_{A}}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{a}}{\theta_{A}}$

-   λ: the wavelength of laser light-   θ_(A): the angle of beam outputted from a luminous point-   D_(A): the total width of beams outputted from all luminous points-   W: the beam width at the position of DMD (at the irradiated surface)-   a: the size of one pixel on DMD-   K: a coefficient determined by beam characteristics, K=1-   M: the magnification of imaging optical system-   t: a required focal depth-   α: an acceptable increased amount of beam diameter

When the interval between luminous points is indicated by P (mm) and thenumber of luminous points is indicated by m, the total width D_(A) ofbeams outputted from all luminous points of the illumination lightsource is expressed by the following formula. For example, the fixedinterval between luminous points may be 0.1 (mm) and the number ofluminous points may be 24.P(m−1)

As shown in FIG. 18, a multi-cavity laser array in which multi-cavitylasers 110 are arranged on a heat block 100 in the same direction as thedirection of arranging luminous points 110 a for each of chips, withpredetermined intervals therebetween, may be used for the illuminatingdevice. It is assumed that the interval between a large number ofluminous points is indicated by P1 (mm), the interval between luminouspoints for adjacent multi-cavity lasers is indicated by P2 (mm), thenumber of luminous points is indicated by m and the number ofmulti-cavity lasers is indicated by N. Then, the total width D_(A) ofbeams outputted from all luminous points for the illumination lightsource is expressed by the following formula.(m−1)N*P1+(N−1)*P2

The DMD 50 has been used as a spatial modulation element in theaforementioned embodiments. However, instead of the DMD 50, reflectiontype or transmission type liquid crystal panels with a plurality ofpixels arranged in a matrix may be used. When a transmission type liquidcrystal panel is used, an optical system may be structured so that laserlight which has been transmitted through the lens system 67 is furthertransmitted through the liquid crystal panel.

As described above, according to the effect of the invention, deep focaldepth can be obtained without providing an autofocus mechanism.

1. An image exposure method in which an exposure head is moved relativeto an exposed surface in a direction orthogonal to a predetermineddirection, comprising the steps of: (a) employing a laser device inwhich luminous points at output ends of optical-fibers of a plurality offiber light sources are arranged, and emitting laser light incident fromincident ends of the optical fibers, from output ends thereof; (b)employing a modulation means having pixels arranged thereon in a twodimensional manner, and changing a modulation state of laser light, inaccordance with a control signal, for each of the pixels; and (c)imaging laser light of which modulation state has been thus changed ontothe exposed surface by an optical system, wherein, in the steps (a) to(c), parameters defined as follows satisfy the following formula,$\frac{D}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{a}}{\theta}$λ: the wavelength of laser light θ: the angle of beam outputted fromoptical fiber that is derived by a numerical aperture (NA) of opticalfiber according to the following formulaθ=sin⁻¹(NA) D: the width of beam outputted from the laser device W: thebeam width at the position of the modulation means (at the irradiatedsurface) a: the size of one pixel on the modulation means K: acoefficient determined by beam characteristics, K=1 M: the magnificationof imaging optical system t: required focal depth α: acceptableincreased amount of beam diameter.
 2. The image exposure methodaccording to claim 1, further comprising the steps of: employing, as themodulating means, a micromirror device in which a large number ofmicromirrors are two-dimensionally arranged on a substrate such thatangles of reflection surfaces of the micromirrors can be variedaccording to control signals; and adjusting parameters defined below soas to satisfy the following formulae, respectively:$\frac{D}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{a}}{\theta}$λ: the wavelength of laser light θ: the angle of beam outputted fromoptical fiber that is derived by a numerical aperture (NA) of opticalfiber according to the following formulaθ=sin⁻¹(NA) D: the width of beam outputted from the laser device W: thebeam width at the position of a digital micromirror device (DMD) (at theirradiated surface) a: the size of one pixel on the DMD K: a coefficientdetermined by beam characteristics, K=1 M: the magnification of imagingoptical system t: required focal depth α: acceptable increased amount ofbeam diameter.
 3. The image exposure method according to claim 1,wherein, when an error in an focal depth direction is indicated by Δz,the following formula is satisfied,Δz=α·M/2{(θ×D)/W+(K×λ)/a}, andt≦Δz.
 4. The image exposure method according to claim 1, wherein theplurality of luminous points are linearly arranged and the angle φ (rad)formed by light outputted from the central luminous point, of saidluminous points, and light outputted from the luminous point placed atthe end portion is represented by the following formula,$\phi = \frac{\theta \times D}{W}$ given that the light reflected by themodulation means is diverged by the divergence angle ψ (rad) because ofdiffraction effect caused by the influence of the size of pixels on themodulation means, the diffraction divergence angle ψ is represented bythe following formula, and $\psi = \frac{K \times \lambda}{a}$ (φ+ψ) inthe predetermined direction is made substantially equal to (φ+ψ) in adirection orthogonal to the predetermined direction.
 5. The imageexposure method according to claim 4, further comprising the step ofmaking the ratio of the longer side direction size of the drive area forthe modulation means to the shorter side direction size of the drivearea for the modulation means, the ratio of the beam width in the longerside direction at the position the modulation means is placed to thebeam width in the shorter side direction at said position, and the ratioof the longer side direction width of beam outputted from the laserdevice to the shorter side direction width of beam outputted from thelaser device substantially equal.
 6. The image exposure method accordingto claim 4, wherein the fiber light source is formed of a high intensityfiber light source.
 7. The image exposure method according to claim 1,wherein an optical fiber in which a core diameter thereof is uniform anda clad diameter thereof at the output end is smaller than a claddiameter at the incident end is used as the optical fiber.
 8. The imageexposure method according to claim 1, wherein the fiber light sourcemultiplexes a plurality of laser lights to make the resultantmultiplexed light incident onto each of the optical fibers.
 9. An imageexposure method in which an exposure head is moved relative to anexposed surface in a direction orthogonal to a predetermined direction,comprising the steps of: (a) emitting laser light from a laser method inwhich a plurality of luminous points are arranged in a predetermineddirection with predetermined intervals therebetween; (b) employing amodulation means having pixels arranged thereon in a two dimensionalmanner, and changing a modulation state of laser light, in accordancewith a control signal, for each of the pixels; and (c) imaging laserlight of which modulation state has been thus changed onto the exposedsurface by an optical system, wherein, in the steps (a) to (c),parameters defined as follows satisfy the following formula,$\frac{D_{A}}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{a}}{\theta_{\Lambda}}$λ: the wavelength of laser light θ_(A): the angle of beam outputted fromluminous points D_(A): the total width of beams outputted from allluminous points W: the beam width at the position of the modulationmeans (at the irradiated surface) a: the size of one pixel on themodulation means K: a coefficient determined by beam characteristics,K=1 M: the magnification of imaging optical system t: required focaldepth α. acceptable increased amount of beam diameter.
 10. The imageexposure method according to claim 9, further comprising the steps of:employing a micromirror method in which a large number of micromirrorsare two-dimensionally arranged on a substrate such that angles ofreflection surfaces of the micromirrors can be varied according tocontrol signals; imaging laser light of which modulation state has beenthus changed by each of the pixels of the micromirror method onto theexposed surface by an optical system; and adjusting parameters definedbelow so as to satisfy the following formulae, respectively:$\frac{D_{A}}{W} \leq \frac{\frac{\alpha \times M}{2 \times t} - \frac{K \times \lambda}{a}}{\theta_{\Lambda}}$λ.: the wavelength of laser light θ_(A): the angle of beam outputtedfrom luminous points D_(A): the total width of beams outputted from allluminous points W: the beam width at the position of a digitalmicromirror device (DMD) (at the irradiated surface) a: the size of onepixel on the DMD K: a coefficient determined by beam characteristics,K=1 M: the magnification of imaging optical system t: required focaldepth α: acceptable increased amount of beam diameter.
 11. The imageexposure method according to claim 9, wherein when the predeterminedinterval between the plurality of luminous points is indicated by P andthe number of luminous points is indicated by m, the total width D_(A)of beams outputted from all luminous points at the laser method isrepresented by the following formula,P(m−1).
 12. The image exposure method according to claim 9, wherein aplurality of multi-cavity laser arrays arranged in the same direction asa direction of arranging luminous points via cavities serve as the lasermethod and when the interval between arranged luminous points isindicated by P1, the interval between arranged cavities is indicated byP2, the number of the luminous points is indicated by m, and the numberof the multi-cavity lasers is indicated by N, the total width D_(A) ofbeams outputted from all luminous points at the laser method isrepresented by the following formula,(m−1)N*P1+(N−1)*P2.
 13. The image exposure method according to claim 9,wherein, when an error in an focal depth direction is indicated by Δz,the following formula is satisfied,Δz=α·M/2{(θ×D)/W+(K×λ)/a}, andt=Δz.
 14. The image exposure method according to claim 9, wherein theplurality of luminous points are linearly arranged, and the angle φ(rad) formed by light outputted from the central luminous point, of saidluminous points, and light outputted from the Luminous point placed atthe end is represented by the following formula,φ=θ_(A) ×D/W given that the light reflected by the modulation means isdiverged by the divergence angle ψ (rad) because of diffraction effectcaused by the influence of the size of pixels on the modulation means,the diffraction divergence angle ψ is represented by the followingformula,ψ=K×λ/a, and (φ+ψ) in the predetermined direction is made substantiallyequal to (φ+ψ) in a direction orthogonal to the predetermined direction.15. The image exposure method according to claim 14, further comprisingthe step of making the ratio of the longer side direction size of thedrive area for the modulation means to the shorter side direction sizeof the drive area for the modulation means, the ratio of the beam widthin the longer side direction at the position the modulation means isplaced to the beam width in the shorter side direction at said position,and the ratio of the longer side direction width of beam outputted fromthe laser method to the shorter side direction width of beam outputtedfrom the laser method substantially equal.
 16. The image exposure methodaccording to claim 14, wherein the fiber light source is formed of ahigh intensity fiber light source.
 17. The image exposure methodaccording to claim 9, wherein an optical fiber in which a core diameterthereof is uniform and a clad diameter thereof at the output end issmaller than a clad diameter at the incident end is used as the opticalfiber.